Key Reminder

I face a problem sometimes forgetting my apartment's key inside the apartment when getting outside. This problem can be solved technologically if the door knob was connected through a technological system to the key hanger. So, when getting into the apartment, the key should be hanged on the key hanger. When holding the knob to get out, the system will send a sign to the key hanger so the hanger light on or ring. In this way, there will be always a reminder of the key when getting out.